The 20-year-old has an £8 million clause in his Panathinaikos contract and the Red Devils are ready to activate it after Sir Alex Ferguson identified the him as the answer to an ageing central midfield.
The Reds are believed to have already opened negotiations with the Athens club about taking Ninis to Old Trafford for next season, and the acquisition of the player would fall in line with the club's current transfer policy of recruiting young, talented players.
20-year-olds Chris Smalling, Bebe and 22-year-old Javier Hernandez have all been added to the Manchester United squad this season.
Ninis was born in Albania but is of Greek descent, and has represented Greece at U21 and senior level. In 64 appearances for Panathinaikos, he has scored six goals.